Electrode-electrolyte interactions on oxide surfaces: the crucial role of surface redox chemistry

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Electrode-electrolyte interactions on oxide surfaces: the crucial role of surface redox chemistry

https://doi.org/10.21203/rs.3.rs-4886343/v1

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The electric double layer (EDL) formed by ions at the electrode-electrolyte interface is a fundamental component of any electrochemical system, ranging from batteries to fuel cells and electrolyzers¹. At the atomic level, the behavior of the EDL varies greatly with the choice of electrode, electrolyte, and conditions^2,3. However, it is thought to follow one general rule analogous to a capacitor^4,5: low electrode potentials lead to a negative electrode surface charge which attracts cations to the EDL, whereas high potentials lead to a positive surface charge and anion attraction. Here, we show that this most basic rule does not apply for the oxide-electrolyte interface. Using an interface-sensitive X-ray absorption spectroscopy (XAS) approach, we show that increasing the electrode potential attracts cations and repels anions at the IrO₂-electrolyte interface. We show that this behavior is driven by the surface redox chemistry of the oxide, which dictates the surface charge of the electrode by modulating the surface electrophilicity. Our findings rationalize the cation-dependent performance trends observed for oxide electrodes^6–9, paving the way to exploit them in electrolyte engineering.

Physical sciences/Chemistry/Electrochemistry

Physical sciences/Chemistry/Catalysis/Electrocatalysis

Physical sciences/Chemistry/Surface chemistry

Electrode-electrolyte interface

Electric double layer (EDL)

Electrocatalysis

In situ X-ray absorption spectroscopy

When a potential is applied to an electrode, this induces a surface charge, which in turn causes a re-arrangement of the ions and water molecules in the near-surface electrolyte¹⁰. This dynamic interfacial electrolyte structure, the EDL, is part of the reaction environment at the electrode surface and therefore has marked effect on the kinetics of electrochemical reactions. Exploiting this electrolyte effect to optimize the performance of electrocatalytic devices is challenging, however, because the relationship between the parameters that are controlled, such as the electrolyte pH and cation^11–13/anion^14–16 composition, and the formed EDL structure is not well understood. This is particularly true for metal oxide electrodes, as here not only the interfacial electrolyte structure is highly dependent on the applied conditions, but also the surface structure of the electrode itself^17–19. In this study, we set out to determine the impact of this dynamic surface structure on the ion-water-electrode interactions and the resulting EDL structure.

To gain direct insights into the ion-water-electrode interactions at the metal oxide-electrolyte interface, we have designed an interface-sensitive electrochemical X-ray absorption spectroscopy (EC-XAS) approach (figure 1a). Sensitivity to the electrode-electrolyte interface is created by making use of: 1) a mesoporous IrO₂ electrode with an extremely high electrode-electrolyte interface area (figure 1b and section 1 in the SI) and, 2) soft-tender XAS with a probing depth smaller than the electrode thickness. In this way, only the electrolyte inside the pores is probed, which is in large part in contact with the electrode. This allows us to selectively investigate the behavior of interfacial ions through their K-edge XAS spectra. We employed this methodology to examine the interactions of Na⁺ and ClO₄^- ions with the IrO₂ electrode in both acidic and alkaline electrolyte, and link this to the observed catalytic activity during the oxygen evolution reaction (OER).

The differences in the OER activity of iridium oxide in acidic and alkaline electrolytes provide a first indication for ion-electrode interactions. Alkaline electrolytes lead to a lower OER activity (figure 1c). Furthermore, the activity in alkaline media depends on the choice of alkali metal cation (figure 1d). These observations suggest that the cations of the electrolyte interact with the OER intermediates under alkaline conditions, thereby affecting the catalytic activity. On the other hand, no cation dependent activity is observed in acidic electrolytes (figure 1e), thus showing the crucial role of the pH in the cation-IrO₂ interactions. These trends are in line with the literature^6,20,21.

In order to examine more closely how Na⁺ ions behave in the vicinity of an electrified IrO₂surface, we stepwise polarized our porous IrO₂ electrode in 0.1M NaOH (pH~13) and 0.1 M HClO₄ + 0.1 M NaClO₄ (pH~1) and recorded Na K-edge X-ray absorption spectra at every potential step. Since these soft XAS measurements only probe the electrolyte inside the pores, the absolute intensity of the Na K-edge allows us to track the concentration of the sodium ions inside the porous electrode. At pH 1, the Na K-edge spectrum exhibits only a weak signal, and there is no significant change in intensity when varying the potential (figure 2a). These findings indicate that at pH 1, the Na⁺ ions are not part of the interfacial electrolyte, in line with the absence of a cation dependent OER activity as shown in figure 1e.

In alkaline electrolyte, Na⁺ ions are strongly attracted to the IrO₂ electrode, as seen from the high intensity of the Na K-edge spectra (figure 2b). Furthermore, it can be observed that the cation-IrO₂ interaction is potential-dependent. Based on the concept of capacitive charging^22–26, the traditional view is that when the electrode potential is increased in the positive direction, positively charged cations will be repelled. However, the intensity of the Na K-edge increases when increasing the applied potential. This implies that with the increase in positive potential, more cations are attracted towards the surface. This result shows that the IrO₂-electrolyte interface does not function as a classical capacitor, indicating that interfacial chemistry beyond basic electrostatics plays an important role in the electrode-electrolyte interactions for oxide electrodes.

To obtain a more quantitative view on the Na⁺ ions interacting with the IrO₂ surface in 0.1 M NaOH, we combined the XAS data and BET surface area analysis to calculate the approximate Na⁺ coverage on the electrode. First, we calculated the Na⁺ ion concentration in the porous volume of the catalyst (figure 2c, left axis), utilizing the edge jump intensity of the Na K-edge spectra (calculation in section 5 of SI). We see that the Na⁺ ion concentration surges to 3 times the bulk concentration at around 1.0 V_RHE and continues to rise at the onset of OER. By incorporating the BET surface area into our calculations, we determined the Na⁺ coverage on the electrode (figure 2c right axis, calculation in section 5.4 of SI). At 1.4 V_RHE the ion coverage reaches 0.8 Na⁺ ion per nm². Comparing this to the area occupied by a hydrated Na⁺ ion (0.2 nm²)²⁷, it is clear that a significant fraction of the surface is covered. This high cation coverage near the electrode surface can be linked to the lower OER activity of IrO₂ in alkaline electrolytes: interfacial ions have been proposed to modify electrocatalytic activity through site blocking, interactions with reaction intermediates, and alteration of the interfacial water structure (water transport/pH buffering effect)^8,11,28.

To verify our conclusions from the XAS analysis, and extend them to other cations, we tracked the mass of the mesoporous electrode in situ using electrochemical quartz crystal microbalance (EQCM). When cations are attracted to the catalyst, they enter the pores and thereby contribute to the observed mass of the electrode. Such a mass increase equates to a decrease in the quartz crystal resonance frequency. In line with the XAS analysis, figure 2d shows a decrease in resonance frequency with increasingly positive potential, confirming that cations are attracted at more positive potentials. The decrease in oscillation frequency (increase in mass) is larger for heavier cations (K⁺, Cs⁺) indicating a response based on the choice of cation. Notably, the mass change exhibits two distinct regimes, the first up to 0.6 V_RHE and the second from 0.6-1.2V_RHE where the latter displays a higher slope. This shows that the cation attraction accelerates at higher potentials, as also confirmed by tracking of the Na K-edge XAS intensity as a function of potential (figure S7b in SI).

To probe the nature of the interaction between the cations and the IrO₂ surface in alkaline electrolytes, we analyzed the shape of the normalized Na K-edge spectra (figure 2e). In the porous volume within the catalyst layer, there are two types of Na⁺ ions: those which reside very close to the electrode in the electric double layer and those located at a larger distance from the surface, i.e., bulk electrolyte that is not under the direct influence of the electrode surface and its electric field. A Na⁺ ion in the bulk electrolyte can be identified by a Na K-edge spectrum with an unequal intensity ratio of the peaks at 1078.2 eV and 1081.5 eV (dark-yellow dotted spectrum in figure 2e). This spectrum represents fully hydrated Na⁺ ions²⁹. Noticeably, our operando spectra deviate from this bulk spectrum, exhibiting a higher 1078.2 eV to 1081.5 eV peak intensity ratio (blue spectra in figure 2e). This increase in peak intensity ratio of the Na K-edge spectrum is the signature of the distortion and opening of the hydration shell of the Na⁺ ion, as shown both experimentally and theoretically^30–32. This indicates that upon interaction with the IrO₂ surface, the Na⁺ ions loose part of their hydration shell. Thus, the Na⁺ ions interact directly with the oxide surface and its active OER sites, which will directly impact the stability of the catalytic intermediates.

In figure 2e, it can be noted that the 1078.2 eV/1081.5 eV intensity ratio is potential dependent. In part, this can be explained by an increasing Na⁺ concentration in the double layer at more positive potentials, because this increases the contribution of adsorbed Na⁺ with respect to Na⁺ in the bulk electrolyte. However, if we subtract the bulk contribution using the spectrum recorded at pH 1, which only contains the bulk contribution, there is still a steady rise in the 1078.2 eV/1081.5 eV peak intensity ratio (figure 2f and section 7 in SI). This implies that when the electrode surface becomes increasingly crowded with Na⁺ ions at high potentials, the hydration shell of the ions is further distorted. To confirm that these strong hydration shell distortions are the result of Na⁺ adsorption rather than Na⁺ intercalation into the oxide lattice, we monitored the IrO₂ lattice oxygen peak at 530 eV in the O K-edge. We observed no changes as a function of time or potential, indicating that the lattice distortion/break-up required for Na⁺ intercalation into the rutile lattice did not occur.

In the following, we will show that the observed strong Na⁺- IrO₂ interactions and their unexpected potential dependence are intimately linked to the surface chemistry of the IrO₂ electrode. To investigate this surface chemistry, we recorded operando O K-edge XAS spectra. The spectral range spanning 527-530 eV in the O K-edge data provides insights into the behavior of surface oxygen atoms in iridium oxide¹⁸. These surface oxygen atoms fall into three categories: 1) H₂O/OH/O bound to one Ir atom at the CUS (coordinatively unsaturated sites), which we term e.g., µ₁-OH, 2) OH/O bound to two Ir atoms at bridge sites, which we term e.g., µ₂-OH, and 3) O atoms bound to three Ir atoms as in bulk IrO₂, which we label µ₃-O. Since the µ₃-O atoms are inert^18,33, we focus on the µ₁- and µ₂- groups here. These groups strongly differ in their O K-edge spectrum, as shown in previous work^18,33–35. This can be used to explain the operando data in figure 3(a-b). The low intensity in the 527-530 eV window observed at ≤0.35 V_RHE in both acidic and alkaline electrolyte is consistent with µ₁-OH₂, µ₁-OH, and µ₂-OH surface groups which display almost no intensity in this potential range. The increasing intensity at 529 eV at higher potentials can be attributed to µ₂-O formation through the deprotonation reaction µ₂-OH à µ₂-O + H⁺ + e^- in acid and µ₂-OH + OH^-à µ₂-O + H₂O + e^- in alkaline electrolyte. Above ~1.2 V_RHE, a broadening towards lower excitation energies is observed, which is attributed to the formation of µ₁-O through µ₁-OH à µ₁-O + H⁺ + e^- in acid and µ₁-OH + OH^-à µ₁-O + H₂O + e^- in alkaline electrolyte. Note that due to acid-base reactions, µ₁-O^- and µ₂-O^- may also be present in alkaline electrolyte, while e.g., µ₂-OH⁺ might be formed in acid. This could explain the differences in observed O K-edge spectra in alkaline and acid media. The common denominator, however, is that the surface becomes strongly electrophilic due to the oxidative deprotonation of the surface -OH groups with the increase in potential.

To understand the influence of the deprotonation reaction on the IrO₂ electrode at increasingly positive potential, we computed the charge on the electrode surface through DFT-RISM^36,37 calculations (details in section 9 of SI). Specifically, we compared the surface charge on a partially protonated (reduced) surface to a deprotonated surface (oxidized) in acidic and alkaline environment (figure 3c). For both electrolytes, we see that the oxidized surface possesses higher negative charge compared to the reduced surface. Based on this, we conclude that the electrophilic µ₁-O and µ₂-O groups at the oxidized surface attract electron density towards the surface (figure 3d). This accumulation of negative charge at the surface is compensated by ions from the electrolyte. In alkaline electrolyte, the overall process is:

In agreement with the experiments, this leads to an increased interfacial Na⁺ concentration in alkaline electrolyte. Furthermore, reaction 1 explains why the cyclic voltammogram of iridium oxides shows a super-Nernstian shift of the surface redox features when increasing the pH^20,38 (see section 8 in SI), because the number of transferred electrons (1-n) is smaller than the number of transferred protons (1).

According to the calculations, the same deprotonation-induced decrease in surface charge should occur in acidic electrolyte. However, no Na⁺ attraction is observed in the pH 1 experiments. This can be rationalized by considering the acid-base chemistry of the surface, which is not fully captured in the calculations, but was already hinted at by the O K-edge spectra. Such acid-base reactions may generate Ir-O-H⁺ groups in acid, leading to a positive surface charge that repels the Na⁺ ions and instead attracts ClO₄^-ions. If oxidative deprotonation makes this positively charged surface less positive at elevated potential, this should repel the ClO₄^- ions through the reaction:

To test this hypothesis, we probed the concentration of perchlorate ions near the IrO₂ surface using Cl K-edge spectra^39,40 and EQCM. Indeed, we see that the Cl K-edge intensity decreases with the increase in applied potential (figure 4a). Similarly, a decrease in mass is observed in EQCM is due to the expulsion of ClO₄^- ions out of the porous electrode at increasingly positive potentials (figure 4b). This confirms our hypothesis and shows the generality of the mechanism uncovered here: whenever the oxide surface is oxidized, this will increase its electrophilicity, drawing negative charge to the surface, attracting or repelling electrolyte ions. Importantly, oxide surface redox is not particular for IrO₂, it is widely observed for oxide materials used in electrochemistry^6,16,41–45. Furthermore, the driving force for ion adsorption is essentially purely electrostatic (figure S11 in SI) and will therefore occur for any ion. Therefore, we expect that the surface redox effect on ion-oxide interactions is an omnipresent phenomenon in oxide electrochemistry.

Acknowledgement: R.V.M. and N.D. acknowledge the Dutch Organization for Scientific Research (NWO) for funding under Grant ECCM.TT.ECCM.001. TEJ acknowledges support from the Laboratory Directed Research and Development program of Los Alamos National Laboratory under project number 20240061. Dr. H. T. acknowledges support from the German Federal Ministry of Education and Research in the framework of the project Catlab (03EW0015A/B). The Helmholtz-Zentrum Berlin für Materialien und Energie is acknowledged for the allocation of synchrotron radiation beamtime at the CAT end station of the EMIL beamline with the proposal number 231-11916. We thank Dr. Sören Selve and Dr. Leyla Kotil of TU Berlin for their assistance with TEM and XRD measurements respectively.

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